The present invention relates to braking the spindle motor in a magnetic disk drive system upon loss of supplied power without damage to the magnetic read/write head.
Magnetic disk drive systems have become widely accepted in the computer industry as a cost effective form of data storage. In a magnetic disk drive system, a magnetic disk rotates at high speed while a magnetic read/write head "flies" slightly over the surface of the rotating disk. The magnetic disk is carried by a spindle drive motor. The magnetic read/write head is suspended over the disk on a spring loaded support arm. As the disk rotates at high speed, the aerodynamic properties of the magnetic head provide a lifting force which allows the head to glide over the disk surface on a cushion of air. The height of the magnetic read/write head over the disk surface is primarily a function of the rate of disk rotation, the aerodynamic properties of the magnetic head assembly (or "slider") and the force provided by the spring loaded support arm.
Two of the most critical periods in determining magnetic head life span occur during "take off" and "landing." Prior to operation, the head rests on an inner track or "landing zone" of the disk. As the disk begins to rotate from an initial, stopped position, the magnetic head is dragged along the surface of the disk. Once the disk reaches sufficient speed, the aerodynamic lift begins to force the magnetic head assembly away from the disk surface, i.e. the head "takes off." The spindle drive motor provides sufficiently large acceleration so that the magnetic head flies after only a very few rotations of the magnetic disk.
During shutdown of the disk drive system, the magnetic read/write head must "land" upon the surface of the magnetic disk. This landing typically occurs in the landing zone along the inner radius of the magnetic disk surface. After power to the magnetic disk drive spindle motor is shut-off, momentum continues to carry the magnetic disk through its rotation. Various friction sources slowly reduce the speed of rotation of the magnetic disk. As the disk rotation slows, the aerodynamic lifting force is reduced and the magnetic read/write head assembly contacts the disk surface in the landing zone area. Once the magnetic head contacts the disk surface, the head is dragged across the surface as momentum continues the magnetic disk rotation.
Magnetic read/write heads used in modern day disk drive systems are typically extremely small and delicate thin film magnetic heads. The dragging associated with take offs and particularly with landings as described above, is a primary source of magnetic head wear. The landing process described above typically produces a longer dragging period for the magnetic head. The prior art has attempted to limit the length of the dragging period by braking the rotation of the spindle motor. One such design is shown in U.S. Pat. No. 4,658,308 issued Apr. 14, 1987 to Sander, Jr. entitled "Method and Apparatus for Retracting Head and Braking Motor of a Disk Drive Device." The Sander, Jr. patent shows circuitry for electrically shorting out the windings in a magnetic disk drive spindle motor following a predetermined time delay after power loss and retraction of the magnetic head onto the landing zone area of the disk surface. Electrically shorting the coils causes the spindle to rapidly stop rotating. Once the motor windings in Sanders are shorted out, the disk drive rapidly stops spinning and the magnetic head looses lift and drops to the disk surface.
Due to size constraints, however, the spindle motor is designed to be as small as possible. This limits the size of the motor windings. Small windings cannot carry large electrical currents without heating and eventually melting. Electrically shorting the windings of the disk spindle motor results in high surge currents due to back EMF induced in the motor coils by the momentum of the rotating disk. (Back EMF is an effect in which an electric motor acts as an electric generator. In the case of a disk drive, the spinning disk causes electric current to flow in the motor windings.) These high surge currents can permanently damage the spindle drive motor by causing one or more of the windings to melt and form an electrical open circuit.
Other prior art approaches to disk drive braking have used mechanical devices such as normally closed electric relays held in the open position by the power supply in the magnetic storage system. Upon loss of power, these relays return to their closed positions and electrically short out the spindle motor windings. This can also cause large surge currents which can damage the motor windings. Mechanical relays have a number of additional design problems including large power draw during operation, large size, high cost and inherent reliability limitations problematic of electromechanical components.
Upon power up of a storage system, it is desirable to rapidly reset the circuitry used to electrically short the windings of the spindle drive motor. Failure to quickly reset the brake circuitry causes either severe loading of the power supply used to drive the spindle motor or an extended delay period during which the braking circuitry slowly releases the spindle motor.